1. Field of the Invention
The invention relates to a sensor arrangement for a calorimetric mass flow meter for measuring the mass flow in a measuring tube, and more particularly, to a sensor arrangement including at least one probe, wherein the probe can be positioned in the cross section of a flow of the measuring tube.
2. Description of Related Art
Arrangements for calorimetric mass flow measurement are known in a plurality of designs from the related art. Calorimetric flow meters are used for determining the flow of fluids, e.g., liquids or gases in piping systems, and are exceptionally suitable for measurements at lower flow speeds. The calorimetric flow measurement is suitable preferably for determining the mass flow since this is immediately proportional to the measuring signal. The heat flow {dot over (Q)} is an essential variable for the determination of the mass flow, which is emitted by a heated sensor to the flowing fluid and is dissipated from the fluid. This dissipated heat flow is consistent with the expended electric calorific output, which can then be used, for example, as a measuring variable. In practice, two methods of measurement are normally distinguished.
A first method is the so-called cool-down method, in which the flowing fluid cools down the heated probe inserted into the cross section of flow of the measuring tube, wherein the temperature difference between the temperature of the fluid and the probe temperature is kept constant. The required calorific output for the constant temperature difference is the variable for determining the mass flow.
A second method is the so-called heat-up method, in which the heat flow emitted by the heated probe increases the temperature of the fluid at a second probe positioned downstream, wherein the increase in temperature ΔT at constant calorific output is a variable for determining the mass flow. In the second method, the increase in temperature ΔT can also be alternatively kept constant by adjusting the calorific output so that the required calorific output can be used as a variable for the mass flow.
A further distinguishing feature for calorimetric mass flow meters known from the related art is the positioning of the probe or, respectively of the probes. Depending on the method of measurement, for example, two probes are arranged next to one another, orthogonal to the direction of flow, wherein one of the two probes is heated and the other measures a reference temperature. A second possibility, which is used in particular for the warm-up method, is that one probe is positioned upstream and one probe further downstream, wherein, in particular, the probe positioned upstream is heated and the probe positioned downstream measures the increase in temperature of the flowing fluid. Alternatively, the arrangement of the heated and un-heated probe can be inverted in respect to the direction of flow, wherein, the probe located upstream measures the reference temperature and the probe located downstream emits a calorific output to the flowing medium.
It is known from the related art that the mounting of the probes on the measuring tube occurs using a threaded connection, in which the probe can be screwed in and positioned in the cross section of the flow. Screwing the probes into the connections of the measuring tube offers the advantage that the probes can be easily removed, for example, for maintenance. However, it is disadvantageous in that the probes and the connections have to be directly adjusted to one another, and thus this type of mounting leads to limited flexibility during an operation, for example when replacing a probe.
The sensor arrangements known from the related art have the further disadvantage that due to the heat transfer between the probes and the measuring tube, an influence occurs on the emitted heat flow or, respectively on the measured temperature, through which the measurement results are falsified and the quality of the measurement results is decreased.